(1) Summary of the Invention
The present invention relates to an improved coaxial cavity type, resonant radiofrequency wave, plasma generating apparatus for producing a beam of ions, free radicals and other excited species from the plasma, which is particularly useful for retrofitting molecular beam epitaxy (MBE) apparatus with small inlet ports. In particular, the present invention relates to an improved plasma generating apparatus, wherein a movable probe and plate are provided in a coaxial cavity to tune the radiofrequency wave to selected modes of resonance and to vary the resonance of the mode in a cavity adjacent to a chamber confining the plasma.
(2) Prior Art
Prior art plasma generating apparatus are described in detail in U.S. Pat. Nos. 4,507,588; 4,585,668; 4,630,566; 4,691,662; 4,727,293; 4,777,336 and 4,792,772 to Asmussen, Reinhard and others. In these apparatus the probe is described to be mounted perpendicular to the longitudinal axis of the chamber for confining the plasma and the coupler for the radiofrequency waves. These apparatus work very well; however, the perpendicular entry of the probe into the cavity of the radiofrequency wave coupler prevents the apparatus from being used in small inlet port apparatus such as MBE apparatus.
Other patents showing narrow beam apparatus are U.S. Pat. Nos. 3,757,518 and 3,866,414 to Bahr; 4,058,748, 4,316,090 and 4,393,333 to Sakudo et al and 4,409,520 to Koike et al. The Bahr patent describes a coaxial cavity while the other patents describe open cavities for the radiofrequency waves.
Other prior art includes the apparatus described by R. M. Fredericks in "An Experimental and Theoretical Study of Resonantly Sustained Plasma in Microwave Cavities", Ph.D. Thesis, Michigan State University (1972), particularly pages 30 to 45; R. E. Fritz, "An Experimental Investigation of the Characteristics of RF Surface Wave Generated Plasmas", M.S. Thesis, Michigan State University, (1979), particularly pages 30 to 38; R. M. Fredericks and J. Asmussen, Proc. IEEE, Vol. 59, No. 2, p. 315, (Feb. 1971); R. M. Fredericks and J. Asmussen, J. Appl. Phys., Vol. 42, No. 9, p. 3647, (Aug. 1971); and J. Rogers and J. Asmussen, IEEE Trans. on Plasma Science, PSIO (1), 11-16, (March 1982). These publications describe apparatus wherein the probe is movable parallel to the longitudinal axis of the cavity and wherein coaxial conductors along the axis are used; however, they do not describe forming a plasma in a chamber adjacent to the cavity confining the microwaves.